The present invention relates to a thin surface light source device comprising a line light source provided on one end edge of a transparent light guiding plate, an end edge reflection layer formed on the other end edge of the light guiding plate, a light diffusion/transmission section and a white back face reflection layer each formed on the back face of the light guiding plate.
The surface light source devices of the present invention are uniform in quality and are suitably used as back-lights for liquid crystal display devices.
Conventionally, thin surface light source devices have either one line light source provided on one end of a light guiding plate or two line light sources provided on opposing ends of the light guiding plate.
The line light source device having one line light source consumes a smaller quantity of power and is lighter than the one having two line light sources, but the former has a disadvantage that the surface emission luminance of the light guiding plate is nonuniform depending on the distance from the line light source. That is, the surface emission luminance of the light guiding plate gradually decreases with an increase in distance from the line light source.
Proposals have been made to overcome the above-described disadvantage. For example, a light diffusion/transmission section has been formed on the back face of the light guiding plate so that the light diffusion/transmission section diffuses light guided to the light guiding plate. A surface light source device according to U.S. Pat. No. 4,985,809 is superior in this respect. A description of this device is set forth below.
The device comprises a line light source provided on one end edge of a transparent light guiding plate, a white side face reflection layer provided on the other end edge of the light guiding plate, and a light diffusion/transmission section and a white back face reflection layer each formed on the back face of the light guiding plate, as shown in FIG. 6. The light diffusion/transmission section is composed of a plurality of fine dot patterns printed on the back face of the light guiding plate. Referring to FIG. 7, the ratio of the area of dot patterns of the light diffusion/transmission section to the whole area of the back face of the light guiding plate (i.e. the light diffusion/transmission section density) gradually becomes greater with an increase in distance from the line light source, in a region from an end edge (a') on the line light source edge of the light guiding plate to a position (b') at which the surface emission luminance of the light guiding plate is lowest, while the ratio of the area of dot patterns of the light diffusion/transmission section to the whole area of the back face of the light guiding plate (i.e. the diffusion/transmission section density) becomes gradually smaller in the region from the position (b') to the end edge (c') on the end edge reflection layer side of the light guiding plate.
That is, the device was invented based on a phenomenon that the surface emission luminance of the light guiding plate gradually decreases with an increase in distance from the end edge on the line light source side while it increases from the position at which the surface emission luminance of the light guiding plate is lowest toward the end edge on the end edge reflection layer side of the light guiding plate under the influence of light reflected by the end edge reflection layer. Accordingly, as shown in FIG. 7, an ideal surface light source device, which provides a uniform surface emission luminance through the light guiding plate as shown by a curve (Z') expressing a resultant surface emission luminance of the light guiding plate, can be manufactured wherein the change in the surface emission luminance of the light guiding plate as expressed by curve (X') is made inversely proportional to a curve (Y') expressing the change in the ratio of dot patterns of the light diffusion/transmission section to the whole area of the back face of the light guiding plate (i.e. expressing the light diffusion/transmission section density).
However, it is very difficult to mass-produce the above-described surface light source device because it is impossible to produce a plurality of line light sources having an equal luminance and a plurality of light guiding plates having an equal light diffusion degree even though each device is strictly quality-controlled. Since the line light source comprises a plurality of parts or substances such as a glass tube, electrodes, a fluorescent substance, mercury, and inactive gas, the luminances of line light sources are different from piece to piece because the qualities of the parts are varied from part to part, and further, it is difficult to assemble a plurality of parts of the same quality. In addition, it is impossible to uniformly print dot patterns on the back face of the light guiding plate with the same quality. The following discrepancies occur from piece to piece in the positions of dots on the light diffusion/transmission section, the sizes of dots, the thicknesses of ink films of dots, the thickness of light guiding plates, the sizes thereof, and the sizes of cut faces thereof. Thus, it is unavoidable that the light diffusion degrees of the light guiding plates will be different from one another.
Therefore, in mass-produced surface light source devices according to U.S. Pat. No. 4,985,809, the following disadvantages occur: Let it be supposed that a curve (Y') expressing the change in the ratio of the area of dot patterns of the light diffusion/transmission section to the whole area is obtained when a position (B') at which the ratio of the area of dot patterns of the light diffusion/transmission section to the whole area of the back face of the light guiding plate is maximum is located at the position (b') where the surface emission luminance of the light guiding plate is lowest; a curve Y'(1) expressing the change in the ratio of the area of dot patterns of the light diffusion/transmission section to the whole area of the back face of the light guiding plate is obtained when the position (B') is located at b'(1) situated between the end edge (a') and the position (b'); and a curve Y'(2) expressing the change in the ratio of the area of dot patterns of the light diffusion/transmission section to the whole area of the back face of the light guiding plate is obtained when the position (B') is located at a position b'(2) situated between the position (b') and the end edge (c'). A curve Z'(1) expressing the surface emission luminance of the light guiding plate and corresponding to the curve Y'(1) is greater than a curve (Z') in parallel therewith in the region from the end edge (a') to the position b'(1), decreases greatly and becomes smaller than that in the region from the position b'(1) to the position (b'), and becomes smaller than that in parallel therewith in the region from the position (b') to the end edge (c'). The curve (Z') expressing the surface emission luminance of the light guiding plate corresponds to the curve (Y'). A curve Z'(2) expressing the surface emission luminance of the light guiding plate and corresponding to the curve Y'(2) is smaller than the curve (Z') in parallel therewith in the region from the end edge (a') to the position (b'), increases greatly and becomes greater than that in the region from the position (b') to the position b'(2), and is greater than that in parallel therewith in the region from the position b'(2) to the end edge (c'). Therefore, the gap between a high luminance point H'(1) and a low luminance point L'(1) of the curve Z'(1) becomes greater with the approach of the position (B') from the position (b') toward the position b'(1). Similarly, the gap between a high luminance point H'(2) and a low luminance point L'(2) of the curve Z'(2) becomes greater with the approach of the position (B') from the position (b') toward the position b'(2). Thus, the qualities of devices according to U.S. Pat. No. 4,985,809 vary from piece to piece. The qualities of the devices can be made to be equal to each other by narrowing the permissible range of the position (B') indeed, but it is necessary to strictly control the quality of the line light source and the light guiding plate.